1. Field of the Invention
The present invention relates to an automatic color control circuit of a color television receiver, and more particularly to a burst signal level detecting circuit which is affected less by noise or the like than conventional color control circuits.
2. Description of the Prior Art
An automatic color control circuit (hereinafter referred to as an ACC circuit) of a prior art color television receiver is constructed as shown in the circuit of FIG. 1. In the circuit of FIG. 1, a color video signal is supplied to a band pass amplifier 1 to obtain amplified chrominance signal which is fed to a color demodulator 2. The chrominance signal from the band pass amplifier 1 is also supplied to a burst gate circuit 3 to create a burst signal Sb which is delivered to phase comparators (synchronous detectors) 4 and 6. The phase comparator 4 is also supplied with an oscillating signal So from a voltage controlled or variable frequency oscillator (hereinafter referred to as a VCO) 5, the free-running frequency of which is a carrier frequency. An output voltage Ec from the phase comparator 4 is fed to the VCO 5 as its control signal, thus forming an automatic phase control (APC) circuit 14. The phase of the oscillating signal So is locked by the burst signal Sb.
The signal So is also supplied to the color demodulator 2 so that red, green and blue color difference signals can be demodulated from the chrominance signal.
The oscillating signal So is also supplied to a phase shifter 7 which produces a signal Sp which is shifted in phase from oscillating signal So by .pi./2. This signal Sp is fed to a synchronous detector 6 which synchronously detects the burst signal Sb and which produces a DC voltage Ed having a level corresponding to that of the signal Sb. The voltage Ed is supplied to the band pass amplifier 1 as its gain control signal (ACC signal). In this manner, an ACC circuit 16 is formed and the level of the chrominance signal from the amplifier 1 is kept constant.
However, in the above ACC circuit 16, if an error exists in the amount in phase shift of the phase shifter 7, the level of the chrominance signal from the amplifier 1 will vary greatly when the phase of the oscillating signal So varies due to the influence of noise or the like. For example, in the phase comparator 4, with the phase of the burst signal Sb being taken as a reference, the relation between the phase .theta.o of the signal So and the output voltage Ec is shown in the graph of FIG. 2A. Thus, the signal So is phase locked to a value of .theta.o=.pi./2 at the stationary condition of Ec equal to zero. Also, in the synchronous detector 6, with the phase of the burst signal Sb being taken as a reference, the relation between the phase .theta.p of the signal Sp and the output voltage Ed is shown in the graph of FIG. 2B. At the stationary condition thereof, the phase .theta.p of the signal Sp from the phase shifter 7 is expressed as follows: EQU .theta.p=.theta.o+.pi./2=.pi.
Therefore, the voltage Ed, obtained from the synchronous detector 6, has a level at the trough or lowermost point on the curve of FIG. 2B corresponding to .theta.p=.pi..
However, as shown in FIG. 2B when the phase .theta.o of the signal So varies by .DELTA..theta. due to the influence of noise or the like, the phase .theta.p of the signal Sp will also vary by .DELTA..theta., resulting in the level of the voltage Ed being changed by .DELTA.E.
The relationship between the ratio of level variation .DELTA.E to phase variation .DELTA..theta. and the phase .theta.p is shown in FIG. 2C, where this relationship is equal to the first derivative of voltage Ed with respect to phase .theta.p. In other words, when the phase .theta.p of the signal Sp is equal to .pi., the variation .DELTA.E of the voltage Ed is small, even though the phase .theta.p is varied by .DELTA..theta.. However, when the phase .theta.p of the signal Sp is varied by .DELTA..theta. such that the phase .theta.p is deviated from .pi., the variation .DELTA.E of the voltage Ed will become large. Such a variation .DELTA.E of the voltage Ed will result in a variation in the level of the chrominance signal.
Accordingly, to prevent large variations in the level of the chrominance signal, the phase shifter 7 is required to have a phase shift of .pi./2. If the phase shifter 7 does not have this value, the phase variation of the signal So will have a great effect on the level variation of the chrominance signal.